Multi-level integrated circuit manufacturing involves metal and insulator film depositions followed by photoresist patterning and material removal steps, all carried out on a workpiece surface such as a wafer surface. Photolithography and etching steps provide many features on the wafer surface such as vias, lines and bond pads. In order to form the interconnect structure, these features need to be filled with a conductive material such as copper. Next, the excess copper, also called overburden, needs to be removed by a technique such as chemical mechanical polishing or electropolishing. These techniques, after the excess material removal step, need to provide a planar wafer surface topography, making it ready again for the next level of processing, which again may involve deposition, photolithographic step and a material removal step. It is most preferred that the substrate surface be flat before the photolithographic step so that proper focusing and level-to-level registration or alignment can be achieved. Therefore, after each deposition step that yields a non-planar surface on the wafer, there is often a step of surface planarization.
Electrodeposition is a widely accepted technique used in IC manufacturing for the deposition of a highly conductive material such as copper (Cu) into the features such as vias and channels opened in an insulating layer on the semiconductor wafer surface. Electrodeposition is commonly carried out cathodically in a specially formulated electrolyte solution containing copper ions as well as additives such as accelerators, suppressors and levelers. These additives along with others such as Cl− ions control the texture, morphology and plating behavior of the copper layer. A proper electrical contact is made to the seed layer on the wafer surface, typically along the circumference of the round wafer. A consumable Cu or inert anode plate is placed in the electrolyte solution. Deposition of Cu on the wafer surface can then be initiated when a cathodic potential is applied to the wafer surface with respect to an anode, i.e., when a negative voltage is applied to the wafer surface with respect to an anode plate.
Standard plating techniques provide bottom-up material growth in submicron size features and to some extent in features with widths up to about 2–3 microns. For larger features standard electroplating yields conformal deposits. Therefore, the resulting copper layer surface display the same topography as the wafer surface at locations where the feature widths are larger than a few microns. Standard plating approaches also deposit large overburden copper that later needs to be removed by techniques such as CMP.
The importance of overcoming the various deficiencies of the conventional electrodeposition techniques is evidenced by technological developments directed to the deposition of planar copper layers. For example, U.S. Pat. No. 6,176,992, entitled Method and Apparatus for Electrochemical Mechanical Deposition, commonly owned by the assignee of the present invention, describes in one aspect an electrochemical mechanical deposition technique (ECMD) that achieves deposition of the conductive material into the cavities on the substrate surface while minimizing deposition on the field regions by polishing the field regions with a pad as the conductive material is deposited, thus yielding planar copper deposits. U.S. Pat. No. 6,482,307 entitled Method and Apparatus for Making Electrical Contact to Wafer Surface for Full-Face Electroplating or Electropolishing, filed on Dec. 14, 2000, describes in one aspect a technique for providing full-face electroplating or electropolishing. U.S. application Ser. No. 09/760,757, now U.S. Pat. No. 6,610,190, entitled Method and Apparatus for Electrodeposition of Uniform Film with Minimal Edge Exclusion on Substrate, filed on Jan. 17, 2001, describes in one aspect a technique for forming a fiat conductive layer on a semiconductor wafer surface without losing space on the surface for electrical contacts.
In such above-mentioned processes, a pad or a mask, which may also be collectively referred to as workpiece surface influencing devices, can be used during at least a portion of the electrodeposition process when there is physical contact between the workpiece surface and the pad or the mask. The physical contact or the external influence by the pad or the mask affects the growth of the metal by reducing the growth rate on the top surface while effectively increasing the growth rate within the features. This aspect is described in U.S. patent application Ser. No. 09/740,701, now U.S. Pat. No. 6.534.116, entitled Plating Method and Apparatus that creates a Differential Between Additive Disposed on a Top Surface and Cavity Surface of a Work Piece Using an External Influence, filed Dec. 18, 2000.
A general depiction of a plating apparatus in which improved anode assemblies such as those of the present invention can be used is shown in FIG. 2. The carrier head 10 holds a round semiconductor wafer 16 and, at the same time, provides rotation and lateral motion to the wafer. Electrical contacts 7 are made to the conductive lower surface of the wafer. The head can be rotated about a first axis 10b. The head can also be moved in the x direction represented in FIG. 2. An arrangement, which provides movement in the z direction is also provided for the head.
A pad 8 is provided on top of an anode assembly 9 across from the wafer surface. The pad 8 may have designs or structures such as those forming the subject matter of U.S. Pat. No. 6,413,388, entitled Pad Designs and Structures for a Versatile Materials Processing Apparatus. U.S. Pat. No. 6,413,403 entitled Pad Designs and Structures with Improved Fluid Distribution. Further, U.S. application with Ser. No. 09/960,236 filed on Sep. 20, 2001, entitled Mask Plate Design, discloses various WSID embodiments. Further, U.S. application with Ser. No. 10/155,828 filed on May 23, 2002, entitled Low Force Electrochemical Mechanical Deposition Method and Apparatus, discloses a WSID structure having a flexible and abrasive top layer attached on a highly compressible layer. Both applications are assigned to the assignee of the present invention.
In a metal deposition process using a soluble anode, it is necessary to minimize contamination of the deposited metal with anode sludge or anode fines. Typically, an anode bag is wrapped around the soluble anode to minimize this sort of contamination. In a conventional copper electrodeposition process, as shown in FIG. 1, an anode bag or filter 150 is wrapped around an anode 152. A suitable space separates the anode 152 from the cathode 154 in the deposition cell 156. Agitation, re-circulation or even filtration of the electrolyte solution 160 may be provided. During routine plating operations, anode sludge builds up in the anode sludge cavity 158 formed by the space between the anode 152 and the bag 150. In the case of Cu plating, excessive anode sludge build-up affects the quality of the deposited metal on the cathode 154 in an adverse manner. In particular, the uniformity of the deposited metal becomes poorer because of changes in the electric field distribution. In addition, the plating voltage increases because of anode polarization. The copper ions are unable to diffuse fast enough through the sludge layer to meet the requirements of the cathode. Moreover, the resulting loss in plating efficiency may cause hydrogen to be plated, or evolve at the cathode. For routine maintenance, the anode 152 is removed from the deposition cell 156 and cleaned or de-sludged before replacement.
Another concern in electrodeposition of electronic materials such as copper deposition for interconnects is the cost. The copper deposition electrolytes such as the sulfate based solutions marketed by companies such as SHIPLEY and ENTHONE need to contain organic additives for best results on wafers. These additives are costly and their depletion from the electrolyte bath needs to be minimized. Accelerators are especially prone to breakdown, although suppressors and levelers also get consumed. There are various mechanisms for organic additive consumption or breakdown in a plating system. The first mechanism is idle flow. In other words, additives may breakdown by just flowing or re-circulating within the system, especially if they are allowed to make physical contact with active surfaces. Active surfaces include but are not limited to metallic surfaces, and even the copper anode itself. Typically, plating systems have a solution tank that has some dosing means for the additives. The additives are dosed into the solution in the solution tank and the dosed solution with additives is pumped into filters and then to the electrodeposition module or modules for processing wafers. Solution from the processing modules is then sent back into the tank, also usually after filtering. This way the solution is continually re-circulated and filtered. During the circulation, additives may break down even if no wafer plating takes place in the process modules.
Another mechanism of additive consumption or breakdown takes place during plating when voltage is applied between the Cu electrode and the wafer surface and there is a plating current passing through the plating circuit. Taking into account the two mechanisms described above we can generally state that additive consumption is a function of the solution flow rate and the charge passed through the solution although there may be other dependences such as temperature and concentration effects.
Reduction of additive consumption is of utmost importance in the interconnect technology not only because these materials are costly, but also because their breakdown products accumulate in the solution and eventually start affecting the quality of the plated copper in a negative way, e.g. start to negatively impact gap-fill capability of the solution. To replenish the bath with fresh solution and to keep the concentration of additive breakdown products under control, a bleed-and-feed approach is commonly used. During bleed-and-feed, some used solution is bled or discarded and approximately the same amount of fresh solution is fed into the tank. The bled solution may optionally be cleaned and re-used later. In any case, using bleed-and-feed keeps accumulation of additive breakdown products under control. During bleed-and-feed, five to twenty percent of the total solution may have to be replaced on a daily basis. It should be appreciated that if the breakdown rate of additives could be minimized, not only the additive usage would be reduced but also the amount of fresh solution used for bleed-and-feed would be minimized. Either way, sizable cost savings and more stable process results can be achieved if additive consumption is minimized.
Several designs of anode assemblies have been disclosed. U.S. Pat. No. 6,261,433 describes an anode for copper plating, where copper electrolyte is pumped through copper particles, which are encased in a porous enclosure. U.S. Pat. No. 6,365,017 describes a plating apparatus comprising an ion exchange film or neutral porous diaphragm dividing the plating bath into a substrate region and anode region. Circulation means are provided to circulate the solution in both regions. U.S. Pat. No. 6,126,798 provides an anode including an anode cup, a filter and ion source material, the anode cup and filter forming an enclosure in which the ion source material is located. U.S. patent application Ser. No. 09/845,262, now U.S. Pat. No. 6,695,962, entitled, Anode Designs for Planar Metal Deposits with Enhanced Electrolyte Solution Blending and Process of Supplying Electrolyte Solution Using Such Designs, filed May 1, 2001 discloses a design that includes two filter elements defining an anode chamber containing the anode, and a blending chamber. The solution emanating from the anode chamber through a primary anode filter mixes with the solution delivered directly to the blending chamber. The mixed solution is then delivered to the substrate surface through a secondary filter.
There is still a need for improved anode assembly designs that provide low additive consumption, avoid anode polarization and at the same time provide high flow of particulate-free electrolyte to the workpiece surface.